The present invention relates to a grid alignment system for portable radiography.
Portable radiography accounts for an increasing proportion of x-ray examinations performed in hospitals. In the University of Chicago hospitals, approximately 50% of all chest radiographs are obtained at the bedside with portable radiographic apparatus. Though the clinical importance of these examinations is beyond question, the image quality is generally inferior to that obtained with fixed radiographic apparatus in an x-ray department. The inferior image quality obtained using portable radiography apparatus is widely recognized, and is a source of concern to radiologists and clinicians. This poor image quality is commonly attributed to intrinsic limitations of portable radiography apparatus, however, it is in fact mainly due to uncontrolled scattered radiation, which fogs the radiograph, reducing contrast and obscuring diagnostic information.
Use of an accurately aligned anti-scatter grid can provide consistently high image quality, but precise alignment of the grid relative to the x-ray source is essential for good results. Such precise alignment is difficult to achieve with conventional manual or "eye-ball" techniques.
For portable radiography, excellent results can be achieved with a 6:1 or 8:1 anti-scatter grid, provided that the x-ray beam energy is no greater than 90 KV, and provided that the anti-scatter grid is accurately aligned with respect to the x-ray source. Referring to FIG. 1, accurate alignment of portable radiographic machine 20 relative to anti-scatter grid 21 and x-ray film 22 is illustrated in position A, while inaccurate alignment is illustrated in position B.
In the case of a conventional lead strip linear anti-scatter grid, alignment is critical only in one dimension, that is, across the grid lines. Moderate angulation error along the direction of the grid lines does not significantly impair image quality. In other words, referring again to FIG. 1, moderate misalignment about a horizontal axis lying within the plane of the page is not so critical, whereas alignment about an axis perpendicular to the plane of the page is critical. Further, with a focused grid, it is also necessary for central x-ray beam 23 of portable x-ray machine 20 to be centered accurately with respect to anti-scatter grid 21. Further, variations in focus distance are important, but mainly affect film density, provided that the portable x-ray machine 20 is properly centered and aligned relative to anti-scatter grid 21. In the case of a two dimensional grid, for example a cross-hatch grid or pinhole grid, alignment is critical in two dimensions.
A system that addresses this alignment problem is presented in U.S. Pat. No. 4,752,948, issued Jun. 21, 1988, and assigned to the same assignee as the present application. The disclosure of U.S Pat. No. 4,752,948 is expressly incorporated herein by reference. While adequately addressing the problem of alignment between x-ray beam and anti-scatter grid in a portable x-ray apparatus, this patented device presents a mechanical system which has proven somewhat difficult to retrofit to existing portable x-ray apparatus, or to use with very ill patients who are unable to cooperate.
Therefore, a need exists for a simple alignment system which can be easily retrofitted to existing portable x-ray apparatus, and which can be used in the most difficult clinical situations.
Commercially available grid cassettes (x-ray film holders incorporating anti-scatter grids) include a 14.times.17 inch (35.6.times.43.2 cm) lead strip grid encased in nylon/plastic material. These grid cassettes are relatively crudely manufactured and are heavy and cumbersome to handle. When the grid lines are oriented vertically, the total transverse dimension of the grid is 14 inches (35.6 cm). Therefore, in heavy-set broad patients, it is necessary to rotate the grid cassette 90.degree. so that the grid lines run transversely, orienting the longer dimension of the grid horizontally. This maneuver, which has been found to be necessary in between 25% and 50% of portable chest radiographs, frequently results in severe misalignment. This is so because the necessary vertical adjustments which must be performed accurately to align the x-ray beam with the grid are more difficult to judge accurately when the grid is transversely oriented In addition, transversely orienting the grid requires the x-ray source and collimator to be rotated, an additional adjustment step.
Therefore, it would also be desirable to maintain the anti-scatter grid orientation in a vertical direction for all patients, while permitting the x-ray film to be oriented either vertically or horizontally facilitating the accommodation of heavy-set broad patients.